1. Field of the Invention
The present invention relates generally to detecting welds or junctures in sheets or strips of material. More particularly, the present invention relates to apparatus and method for detecting welds in steel sheets or strips.
2. Description of the Related Art
In the manufacture of steel strips, steel sheets, and other metallic and nonmetallic strips and sheets, it is common to weld or join two pieces of strip or sheet together. While subsequent processing, such as hot or cold rolling of steel strips or steel sheets, may obscure welds, there remains changes in thickness at the weld or juncture that are detrimental to the production of uniform products.
More particularly, a thickened area at a weld or juncture often results in irregular drawing or even fracturing of a part that is being deep drawn. The problem of thickened areas at welds or junctures becomes particularly critical when a sheet being drawn has additional costs, such as plating or decorating, invested in previous operations on the sheet to be drawn.
The prior art has addressed the problem of detecting flaws or imperfections in strips and sheets. A number of prior art patents have been directed to the use of eddy currents to detect weld seams. An early example of this type of prior art is Harmon, U.S. Pat. No. 2,832,040.
Detecting of weld seams has been accomplished by measuring thicknesses of sheets or strips, that is by measuring changes in thicknesses. An early device that used eddy currents for measuring thicknesses of sheets is taught by Lenehan in U.S. Pat. No. 2,443,661: and continuing development in the art is seen in Charpentier, U.S. Pat. No. 4,757,259 and Lonchampt et al., U.S. Pat. No. 4,727,322.
Not only have eddy current devices been used to detect weld seams, but also they have been used to detect various flaws. One such device is taught by Mandula, Jr., et al. In U.S. Pat. No. 3,263,809; and another is taught by Harmon in U.S. Pat. No. 3,497,799.
However, changes in thicknesses of sheets or strips at welds or junctures are relatively small, especially for sheets or strips with gauges having thicknesses in the range of 0.006 to 0.010 inches (0.152 to 0.254 millimeters). Thus variations in the output of position sensors, or thickness sensors, have been relatively small; so that, even with amplification, it has been difficult to accurately sense weld seams. This has been an especially difficult problem because of the fact that seam-weld detection is typically accomplished with sheet or strip velocities in the order of 600 feet per minute (3.048 meters/second).